SUBMARINE GROUNDWATER DISCHARGE OF NUTRIENTS FROM URBAN COASTAL AQUIFERS IN NEW YORK CITY

Excess input of nutrients to coastal waters by submarine groundwater discharge could damage coastal ecosystem by causing algal blooms and red tides. A hydrologic and geochemical study was conducted for a total of 3 study sites including Great Kills Park, Miller Field, and Verrazano-Narrows Bridge sites along the sandy coast in Staten Island, NY. Permeability of intertidal zone sediments (n=26) determined by constant head permeability tests showed that permeability for sediments at 0-0.5 m depth is similar for all 3 sites, with average permeability ranging from 3.26×10^-7 cm² to 4.18×10^-7 cm², whereas permeability for sediments at 0.5-1.0 m depth is an order of magnitude lower for Great Kills Park site (average 3.98×10^-8 cm²) than Miller Field and Verrazano-Narrows Bridge sites (average 3.66×10^-7 to 3.88×10^-7 cm²).

Saline groundwater samples were collected in the intertidal zone at the Verrazano-Narrows Bridge site, along a presumed flow path (22-m transect) of groundwater discharging to the Lower New York Bay. Water samples were analyzed in the field using CHEMetrics test kits and Hach DR/890 Colorimeter. Groundwater collected at 2 m depth in the most landward area is characterized with low dissolved oxygen (average 2.5 mg/L), relatively low EC (4.85 ms/cm) and salinity (1.6 ppt), as well as low ammonia (average 0.02 mg/L), while dissolved nitrate and phosphate were on average 2.4 mg/L and 0.59 mg/L, respectively. Along the presumed groundwater flow path toward the shore, DO increased from 2.5 mg/L to 6-8 mg/L and salinity increased from 1.6 ppt to 14.5 ppt, while phosphate decreased from 0.59 mg/L to 0.20 mg/L. From the upland to the shore, nitrate increased from 2.4 mg/L to 9.4 mg/L, and then decreased to 1.3 mg/L, while ammonia remained low at 0-0.06 mg/L. For seawater collected near the shore, salinity was 17.2 ppt, DO was 6 mg/L, nitrate concentration was below 0.4 mg/L, and phosphate concentration was 0.20 mg/L, while ammonia concentration was 0.15 mg/L, which was higher than that (0-0.06 mg/L) in saline groundwater. The preliminary field results suggest that nutrients such as nitrate and phosphate are added to the Lower New York Bay by groundwater discharge from saline coastal aquifers in Staten Island, New York.